Differential transformer



Aug. 18, 1970 w. w. "WAHLGREN DIFFERENTIAL TRANSFORMER 2 Sheets-Sheet l Filed Sept. 16, 1968 n w e S 2 T 1 n f w w u m fA e x F M W Augt 1970 w. w. WAHLGREN 3,52

DIFFERENTIAL TRANSFORMER Filed Sept. 16, 1968 2 SheetsSheet 2 /mvmma Wallace W Wahlgren United States Patent Oifice 3,525,063 Patented Aug. 18, 1970 3,525,063 DIFFERENTIAL TRANSFORMER Wallace W. Wahlgren, Oakland, Calif., assignor to The Rucker Company, Oakland, Calif, a corporation of California Filed Sept. 16, 1968, Ser. No. 760,023 Int. Cl. H011? 27/28 US. Cl. 336-96 4 Claims ABSTRACT OF THE DISCLOSURE Differential transformer having a magnetic core comprised of a plurality of superposed, generally U-shaped laminations with alternate laminations reversed end for end to provide a core with a generally four-sided rectangular configuration and having a rectangular opening therein to provide a magnetic core having a substantially uniform cross-sectional area. At least one secondary winding is mounted on the core and at least two substantially identical primary windings are wound on the core so that they have the same geometrical relationship to the secondary winding whereby the leakage path reluctance between each of the primary windings and the secondary winding is substantially identical. The method is for the steps of making the diiferential transformer.

BACKGROUND OF THE INVENTION In the production of ground fault circuit interrupters, there is a need for dilferential transformers for the protection of personnel from electric shock hazard. There is a need for such diiferential transformers which are well balanced and which are capable of carrying relatively high currents. Toroid type of dilferential transformers have heretofore been provided; however, it has been found that with such differential transformers, it is very difficult to obtain the high degree of balance which is required and also the uniformity of manufacture which is required and which will still retain the required sensitivity regardless of the current rating and the system voltage applied to the ditferential transformer. There is, therefore, a need for a new and improved differential transformer.

SUMMARY OF THE INVENTION AND OBJECTS The differential transformer consists of a magnetic core which is comprised of a plurality of superposed,

generally U-shaped laminations to form a magnetic core which is generally rectangular in configuration and which has a rectangular opening therein. The magnetic core has a substantially uniform cross-sectional area to provide a relatively uniform flux density. At least one secondary winding is wound on the core. At least two substantially identical primary windings are also wound on the core and on the secondary winding in such a manner that the several primary windings each have the same geometrical relationship to the secondary winding so that the leakage path reluctance between the secondary winding and each of the primary windings is substantially identical.

In general, it is an object of the present invention to provide a differential transformer which is capable of carrying normal load currents, either balanced or unbalanced, without appreciably aifecting the sensitivity of the different transformer to ground leakage currents.

Another object of the invention is to provide a dif ferential transformer of the above character which is capable of carrying fully rated load currents without overheating.

Another object of the invention is to provide a differential transformer of the above character in which the voltage drop is a very small percentage of the system voltage.

Another object of the invention is to provide a differential transformer of the above character in which the primary windings have a substantially identical geometrical relationship to the magnetic core and to the secondary winding.

Another object of the invention is to provide a differential transformer of the above character which is capable of withstanding fault and short circuit conditions in the power circuit for brief periods of time.

Another object of the invention is to provide a method for making a differential transformer of the above character.

Additional objects and features of the invention will appear from the following description in which the preferred embodiment is set forth in detail in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an isometric view showing the winding of the liner utilized in the differential transformer and being wound on a mandrel.

FIG. 2 is an isometric view showing the secondary winding of the differential transformer wound on the liner.

FIG. 3 is an isometric view showing insulating material wrapped about the secondary winding.

FIG. 4 is an isometric view showing the Faraday shield mounted on the insulating material surrounding the secondary Winding.

FIG. 5 is an isometric view showing the insulating material found about the Faraday shield.

FIG. 6 is a view showing the manner in which the primary windings are formed on a mandrel.

FIG. 7 is an exploded view showing the manner in which the differential transformer is assembled.

FIG. 8 is an isometric view showing further details in the construction of the differential transformer.

FIG. 9 is a bottom plan view of a differential transformer incorporating the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The preliminary steps for making a differential transformer of the type incorporating the present invention are shown in FIGS. 1-7. As shown in FIG. 1, a core tube 10 of a suitable insulating material, such as kraft paper, is placed on a mandrel 12. This core tube 10, which is generally square in cross-section as can be seen in FIG. 1, is wrapped with a sheet 13 of high quality insulating material as shown in FIG. 1. As soon as this is completed, a part of the secondary winding 14 of relatively fine copper Wire is Wound over the sheet 13 as shown in FIG. 2, and the ends of the insulated Wire are connected to insulated conductors 16 and 17. Thereafter, as shown in FIG. 3, this part of the secondary winding is covered by an additional sheet 18 of high quality insulating properties wound about the part of the secondary winding. Shielding means in the form of a suitable material, such as a thin copper sheet 21, is wrapped over the sheet 18, as shown in FIG. 4, and is connected to an insulated conductor 22 to provide. a Faraday shield for the part of the secondary winding. A sheet of insulating material 23 is then Wrapped over the shielding means formed by the sheet 21 as shown in FIG. 5 to provide a completed secondary winding assem'bly.

After the steps shown in FIGS. l-5 have been completed, parts of three primary windings 24 are prepared by winding three separate, relatively large insulated conductors simultaneously onto a mandrel 26 to provide a 3 primary winding assembly. The ends of each of the parts of the primary windings serve as leads 25. The turns of each part of the primary windings are disposed side by side to provide primary windings with at least one turn in each part, and preferably more than one turn, and with a precise geometrical configuration.

Assemblies of the type shown in FIGS. 5 and 6 are removed from their associated mandrels and are assembled as shown in FIG. 7 to provide two separate coils 27. The assembly shown in FIG. 5 is inserted within the assembly shown in FIG. 6 to provide each of the coils 27. Each of the coils 27 is then wound with a sheet 28 of insulating material to complete the coil.

After the coils 27 have been assembled, a magnetic core 29 is provided. The magnetic core 29 is comprised of a plurality of laminations 31 which can be characterized as Type DU laminations. The DU lamination is a type well known to those skilled in the art and generally consists of a U-shaped member 32 which is provided with a pair of leg portions 32a and 32b and an integral base portion 320. The base portion 32c has a width which is approximately twice as great as the width of the leg portions 32a and 32b. The DU laminations are made of suitable material, such as an 80% nickel alloy. Such an alloy is used to achieve high permeability and also to achieve a low core loss.

The laminations 31 are inserted into the holes 11 in the coils 27 in such a manner that the laminations are superposed or stacked one above the other with alternate laminations being inserted from opposite sides of the primary windings to provide a magnetic core 29 that serves to provide a rectangular flux path and which has a rectangular window 33. The DU laminations utilized provide two laminated legs on which the two coils 27 are disposed as shown in FIG. 7. The length of the laminated legs is such that the coils 27 occupy from 90-95% of the space in the window 33. The coils 27 are interconnected to provide the secondary winding and at least two primary windings for the differential transformer. Thus, the two parts of the secondary winding forming a part of the two coils 27 are connected in series with the leads 17 being tied together as shown in FIG. 8 and with the leads 16- being available for connection to the outside world. The leads 22 are interconnected as shown in FIG. 8 and are connected to a lead 36. The primary leads 25 are then positioned in the manner shown in FIG. 9.

As pointed out previously, the coil 27 will occupy from 90-95% of the space in window 33-. As also can be seen, the coils 27 extend substantially the entire length of the laminated legs of the core 29. It is desirable that each part of each winding in the core extend through substantially the entire width or length of the coil 27 and thus along substantially the entire length of the leg on which the coil is mounted. Thus the part of the secondary winding which forms a part of the coil 27 can be formed of more than one layer, but each layer should extend the entire length of the core. Each part of the primary winding is preferably formed in a single layer and also should extend the entire length of the core.

When the cores have been assembled in the manner shown in FIGS. 8 and 9, the two series connected parts of the secondary winding form 'a complete secondary winding in which the two parts are substantially equal to each other and thus the secondary winding is distributed uniformly between the two legs of the core 29. Similarly, the parts of the primary windings of the two coils 27 are interconnected so that each of the primary windings is uniformly distributed over the two legs of the core.

The geometrical relationships explained above are important in order to achieve the degree of balance which is required in the differential transformer. Each primary turn and each fraction thereof of each of these several primaries must have the same geometric relationship to the secondary turns in order that the leakage path reluctance of all the windings will be the same. In other words,

the leakage path reluctance should be as close as possible for each fractional part of the coupled primary and secondary windings. For this reason it is preferable that there only be one layer for the primary windings and one or more full layers for the secondary winding. It is also for these reasons that the leads 25 of the parts of the primary winding 24 are positioned in the precise manner shown in FIG. 9. It can be seen that the leads 25 from each of the coils 27 are brought out in the same manner so that no unbalance occurs because of the manner in which the primary leads are brought out for the primary windings.

After the primary leads 25 have been properly positioned as shown in FIG. 9, the entire assembly can be placed in a case 4 1 of a suitable material and then encapsulated or potted therein by suitable epoxy resin 42 to hold all of the leads in their desired positions. Prior to encapsulation, spacers 38 of insulating material can be provided between the windings on each leg to prevent movement of the windings on the legs.

Operation of the differential transformer may now be briefly described as follows. The dilferential transformer has been utilized for ground fault detection systems of the type described in US. Letters Pat. No. 3,213,321 and has been found to operate very satisfactorily. By way of example, the differential transformer has been capable of carrying normal load currents up to the full rating of the device as, for example, 70 amperes, either balanced or unbalanced, without appreciably affecting the sensitivity requirements of tripping out the breaker quickly with insulation leakage currents to ground in excess of 3 milliamperes but not more than 5 milliamperes regardless of the current rating of the device and the system voltage.

It can be seen that the differential transformer is a current transformer or a series connected transformer having two or more substantially identical primary windings and one or more secondary windings. Both the outgoing and return currents flow in the primary windings and for that reason the current returning through one primary winding should be exactly equal to the outgoing current flowing in the mate primary winding. When this is the case, the ampere turns of magnetizing force created by each primary winding is equal and opposite and, therefore, cancel each other for a net resultant magnetizing force of zero. However, if for any reason the currents in each primary winding are not equal and opposite in direction, then a magnetizing force exists which is based on the difference between the magnitudes of the two currents. These ampere turns then magnetize the core and, in turn, induce voltage into the secondary winding which, in turn, may be used to actuate a device. It should be understood that both single phase two and three wire systems and also three phase four wire systems operate in the same manner even though the going and return currents may be distributed unequally among the wires. The sum of the outgoing currents and the return currents must always algebraically and instantaneously equal zero unless there is leakage in the load system. However, for this to be true, all the conductors must have a primary winding of equal turns in the differential transformer.

In order to achieve the desired characteristics in the differential transformer, it is necessary to reduce the leakage inductance among the several primary windings to a minimum and this is accomplished by eliminating as much as possible any spacing between the primary windings. Thus, as shown, the three primary windings are wound simultaneously. This is important because the magnitude of the primary current is very large in comparison to the 5 milliamperes of differential current within which the secondary winding does not occur simply due to unbalanced primary windings.

The differential transformer must be capable of withstanding fault and short circuit conditions in the power circuit for brief periods without unduly affecting the differential transformer of the associated components when the differential transformer is utilized in a ground fault detection system.

The DU type lamination hereinbefore described is particularly desirable for the present differential transformer because when the alternate laminations are reversed end for end, there are no butt joints. The flux lines complete the circuit by passing to the adjacent laminations over the large area 320 adjoining the leg portions 32a and 32b. For this reason, the effective reluctance for the magnetic circuit is small. In addition, the flux density throughout the core is substantially uniform and no part of the core has a high flux density.

The construction utilized for the differential transformer makes it possible to manufacture very precise secondary and primary windings by the use of mandrels. This also makes it possible to produce differential transformers in quantity and which are capable of meeting exacting specifications.

The primary windings are wound separately on a separate mandrel and not on the secondary windings so they do not place mechanical strain or stress on the very fine wire utilized in the secondary winding but primarily to insure uniformity of coupling between the primary and secondary windings.

It is apparent from the foregoing that there has been provided a differential transformer which is capable of meeting very exacting specifications and which can be produced in quantity so that they can be utilized in critical applications as, for example, in ground fault detection.

I claim:

1. In a differential transformer, a magnetic core comprising a plurality of superposed generally U-shaped laminations wherein alternate laminations are reversed end-toend to provide a magnetic core which is generally rectangular in configuration and which has a window therein, the magnetic core having pair of spaced legs and having substantially uniform cross-sectional area to provide a relatively uniform flux density, a secondary winding formed of at least two parts with one of the two parts mounted on each of the legs of the core, said two parts being substantially identical to each other, and at least two primary windings on the legs, each of said primary windings being formed of at least tWo parts with one of the two parts mounted on each of the legs, the primary winding parts on each leg being disposed in a single layer with the turns of each part being disposed side by side, said primary and secondary windings being positioned in such a manner that each of the primary windings has substantially the same geometrical relationship to the magnetic core and to the secondary windings.

2. A differential transformer as in claim 1 wherein said primary windings have leads extending therefrom and wherein said leads are arranged so that they have the same physical relationship to the secondary windings and also to the magnetic core together with means for potting said leads to retain them in the desired relationships.

3. A differential transformer as in claim 1 wherein said primary windings and said secondary windings are precisely wound.

4. A differential transformer as in claim 1 wherein said parts of the secondary windings and the primary windings on each leg are formed into a coil with the secondary part extending the length of the coil and disposed within the parts of the primary winding and wherein the parts of the primary windings also extend substantially the entire length of the coil.

References Cited UNITED STATES PATENTS 687,048 11/1901 Moody 336-184 XR 729,748 6/1903 Frank 336-184 XR 840,150 1/1907 Moody 336-184 XR 1,308,448 7/1919 Sandell 336-184 XR 1,554,664 9/1925 Stephens 336-184 3,041,565 6/1962 Bradbunn et al. 336-217 3,339,013 8/1967 Gainer et al. 336-96 XR THOMAS J. KOZMA, Primary Examiner US. Cl. X.R. 

